Observations of giant fountains of multimillion-degree gas in the outer atmosphere of the Sun may have solved a long-standing solar mystery: why is its outer atmosphere so hot?

Observations made with the Trace (Transition Region and Coronal Explorer) satellite may have found the much sought-after location of the heating mechanism that makes the corona 300 times hotter than the Sun's visible surface.

Scientists discovered the important clue while observing immense coils of hot, electrified gas, known as coronal loops, which have been seen in unprecedented detail by Trace.

Astronomers also hope to use the solar corona studies to better understand other stars.

Old questions

"The mysterious energy source that makes the Sun's atmosphere so incredibly hot has been an enigma for more than 70 years, and before we discover what it is, we needed to learn where it is," said Dr Markus Aschwanden, of the Lockheed-Martin Solar and Astrophysics Laboratory, in Palo Alto, California.

A forthcoming paper in the Astrophysical Journal, Locating the Source of Coronal Heating, is seen as an important advance in understanding the puzzle.

According to Dr Aschwanden, "we are excited that solar observatories like Trace are allowing us to resolve the hidden events occurring in the atmospheres of stars".

The new observations show that the location of the unidentified energy source is within about 16,000 km (10,000 miles) of the Sun's visible surface.

Dynamic interface

The gas fountains seen in the Trace images form arches hundreds of thousands of kilometres in height and are capable of surrounding 30 Earths. As gas emerges from the solar surface, it is heated and rises, then cools and crashes back to the surface at more than 100 kilometres per second (60 miles per second).

The conventional theory assumes that the loops are heated evenly. However, Trace observations show that most of the heating must occur at the base of the loops, near where they emerge from and return to the solar surface.

"Since a loop loses heat most rapidly from its base, most of the heat must also be going in at the bases for the loop to be at a uniform temperature," said solar researcher Dr Karel Schrijver. "If this were not so, the lower parts would have been much cooler than the tops, which do not lose heat as quickly."

Trace was launched in April 1998 to investigate the so-called transition region of the Sun's atmosphere, the dynamic interface between the relatively cool surface and hot outer atmosphere.